Mica in a sedimentary feature as evidence of humid conditions for a classic desert in Paraná Basin, southern Brazil.

Abstract

In Paraná Basin, oriented mica appears into horizontal layered fine sediments feature defined as geodes with sediments. Mica is identified as biotite in a volcano-sedimentary context of Botucatu and Serra Geral formations. Throughout the accomplishment of multiple analyses such as petrography, rock chemical evaluation, Rare Earth Elements (REE) values and also the Chemical Index of Alteration (CIA) some contributions for local paleoenvironment and paleoweathering for the classic Botucatu paleodesert is proposed. Since biotite is not regular in sediments of a desert environment, the mica presence suggests that it was transported by aqueous input and deposited within the provided spaces. The arrangement of these sediments as planar cross-stratification also confirms that this process is water dependent, otherwise the preservation of the layers’ horizontality would not be preserved. The REE results of negative Ce (Cerium) anomaly in some features, geochemically implies the contribution of water column during deposition. Some geodes are intensely silicified, and in these, there is a relative preservation of Ce attributed to an intense siliceous cementation, avoiding the oxidation of Ce. Despite some differences in Ce values, these features hold similar REE patterns and CIA values indicating similar provenance for the geodes’ sediments and they were probably deposited under similar chemical conditions. The Ce difference between the slightly silicified and intensely silicified geodes shows local conditions for the values, influenced by the presence of water presence and silicification.